Recently, an oxyfuel combustor has been reviewed as one of techniques for reducing emission of carbon dioxide (CO2) which is said to be one of factors for global warming, and attention has been attracted to, for example, a coal-fired boiler for oxyfuel combustion of pulverized coal. It has been conceived in the coal-fired boiler that oxygen in lieu of air is used as an oxidizing agent to produce combustion exhaust gas mainly composed of carbon dioxide. The exhaust gas mainly composed of carbon dioxide is compressed and cooled into liquefied or compressed carbon dioxide which is transported to and disposed at a destination. As an example of the disposal, it has been conceived that the liquefied carbon dioxide is stored in the ground. An exhaust gas treatment device for such coal-fired boiler for oxyfuel combustion is disclosed, for example, in Patent Literature 1.
As shown in Patent Literature 1, the oxyfuel combustion of coal by the coal-fired boiler brings about exhaust gas containing, in addition to carbon dioxide (CO2) as main substance, impurities derived from coal supply stock such as nitrogen oxides (NOx), sulfur oxides (SOx), hydrogen chloride (HCl), mercury (Hg) and dust.
For removal of sulfur (S) derived from the coal supply stock and admixed in the exhaust gas from, for example, a coal-fired boiler for oxyfuel combustion, Patent Literature 1 provides a spray-column-type, packed-column-type or other so-called wet desulfurizer used in a conventional air-fired boiler or the like to remove sulfur oxides (SOx). Moreover, for removal of nitrogen (N) derived from the coal supply stock and admixed in the exhaust gas from, for example, the coal-fired boiler for oxyfuel combustion, a catalyst-type or other denitrator is provided to remove nitrogen oxides (NOx). Carbon dioxide thus free from the impurities is guided to and compressed by a compressor.
Among the above-mentioned impurities, sulfur oxides (SOx) may be contacted with and dissolved in water into sulfuric acid and hydrogen chloride (HCl) may be dissolved in water into hydrochloric acid, so that such water-soluble sulfur oxides and hydrogen chloride can be removed through contact with water.
Among the nitrogen oxides (NOx) as the above-mentioned impurities, nitrogen dioxide (NO2) may be contacted with and dissolved in water into nitric acid to become removed. However, the exhaust gas from the coal-fired boiler has less oxygen (O2) so that nitrogen (N) exists substantially in the form of nitrogen monoxide (NO) which is water-insoluble and thus is unremovable by water spraying or the like.
The above-mentioned sulfuric acid, hydrochloric acid and nitric acid are known to corrode parts in the exhaust gas treatment device; mercury, which is trace metal, is known to hurt low-temperature aluminum members constituting a heat exchanger. Thus, preferably, these impurities are to be removed at early stages. There is also a problem that admixture of the impurities into the exhaust gas lowers a purity degree of the carbon dioxide, which makes troublesome the liquefaction of the carbon dioxide through compression and cooling and thus requires larger-sized equipment therefor. Further, it is feared that any sulfur oxides admixed in carbon dioxide liquefied and stored in the ground may react with calcium in the ground to harm sealability of the storage. Thus, in a coal-fired boiler for oxyfuel combustion or other system where produced is exhaust gas mainly composed of carbon dioxide which is to be disposed, it is extremely important to remove impurities in the exhaust gas.
It has been known in the wet desulfurizer as mentioned in the above that water-soluble sulfur oxides and hydrogen chloride as well as dust are removed and nitrogen oxides are partly removed and that mercury, which is inherently low in content, is slightly removed. Moreover, it has been conceived that if mercury in the exhaust gas is still high in concentration even after the above-mentioned exhaust gas treatment is conducted, a mercury-removing column is arranged to remove the mercury by adsorbent or the like.
However, in Patent Literature 1, the impurities in the exhaust gas are removed by both the spray-column-type, packed-column-type or other wet desulfurizer and the catalyst-type or other denitrator arranged in the circulation line. Thus, the desulfurizer and the denitrator for removal of the impurities in the exhaust gas becomes extreme large in size, complicated in structure and increased in installation cost.
Patent Literature 2 discloses a method for purifying gas including CO2, using an absorptive purification unit. Patent Literature 2 with (a) a pretreatment process for removal of impurities in the gas from an oxyfuel combustor, (b) a process for compression of the gas from the process (a) into a pressure between 10 and 50 bar and (c) a process for recovery of the purified CO2-enriched gas in a liquid, gaseous or supercritical state, has an adsorbent bed (adsorptive purification unit) with adsorption character capable of removing water at least partly in the presence of at least either of NOx or SOx at between the processes (a) and (b) or downstream of the process (b).
However, Patent Literature 2 also becomes complex in structure and increased in installation cost since the adsorptive purification unit is provided to remove impurities in exhaust gas from oxyfuel combustion. Moreover, the adsorptive purification unit is troublesome in maintenance.
In order to overcome the above-mentioned problems, an impurity removal system has been proposed which comprises a compressor for compression of exhaust gas composed of carbon dioxide from an oxyfuel combustor into a target pressure for disposal thereof. Impurities in the exhaust gas is removed by drain produced by cooling of the exhaust gas compressed by the compressor through a cooler. Further, an alkaline aqueous solution is supplied to an exhaust gas entry side of the cooler arranged downstream of the compressor, thereby enhancing impurity removal effect. According the impurities removal system, impurities can be removed, utilizing the compressor and the cooler provided for liquefied or compressed transportation of the carbon dioxide, thereby attaining substantial simplification of the device.
In the impurities removal system, the compression of the exhaust gas by the compressor facilitates oxidization of the impurities in the exhaust gas; the oxidized impurities tend to be dissolved in the drain produced by cooling through the cooler downstream, whereby the impurities are removed together with the drain. In this case, spray of the alkaline aqueous solution to the exhaust gas entry side of the cooler makes it easy to absorb the oxidized impurities in the exhaust gas by the alkaline aqueous solution having a high pH value, thereby enhancing the impurity removal effect.
General state of art pertinent to the impurities removal system is disclosed, for example, in Patent Literatures 3, 4, 5 and 6.
Patent Literature 3 discloses an exhaust gas treatment system for an oxyfuel combustor with a front impurity removal device and at least one rear impurity removal device. The front impurity removal device comprises a compressor for compression of exhaust gas from the oxyfuel combustor to make water-soluble the impurities in the exhaust gas and a cooler for cooling of the exhaust gas compressed by the compressor to condense water therein to discharge drain with the impurities dissolved. The or each rear impurity removal device comprises a rear compressor for compression of the exhaust gas at a pressure higher than that in the compressor and a rear cooler and serves to discharge drain.
Patent Literature 4 discloses an exhaust gas washing treatment system comprising a quenching/reaction section for cooling of exhaust gas from garbage disposal equipment while neutralizing the same using an aqueous solution containing an alkaline component, and a humidity reducing section for further cooling of the exhaust gas neutralized by the quenching/reaction section. The quenching/reaction section is provided with a cooling-gas introducing device for cooling of the exhaust gas.
Patent Literature 5 discloses an exhaust gas treatment column in which, in a first stage of a column body, exhaust gas is gas-liquid contacted with treating water to remove harmful components in the exhaust gas and, in a second stage, the exhaust gas is cooled and washed to discharge the treated exhaust gas through an exhaust gas outlet on a upper portion of the body. In the exhaust gas treatment column, part and remaining part of the exhaust-gas treating water in the second stage are led to waste-water and circulation tanks, respectively. The treating water stored in the circulation tank is led through a treating-water circulation piping to the first stage and is utilized for the exhaust gas treatment while alkaline liquid is fed to the circulation tank to adjust pH of the treating water in the circulation tank.
Patent Literature 6 discloses a CO2 chemical absorption system comprising a CO2 chemical absorption apparatus and an absorbing-liquid reclaimer. In the CO2 chemical absorption apparatus, CO2 in exhaust gas is contacted with amine-based absorbing liquid in an absorption column, the absorbing liquid having absorbed CO2 being heated in a reclamation column to release CO2, the exhaust gas made free from CO2 being cooled to separate condensed water therefrom, the condensed water being circulated to the reclamation column. In the absorbing-liquid reclaimer, the amine-based absorbing liquid is withdrawn from the reclamation column, a heat-stable salt accumulated in the absorbing liquid being removed by distillation, resultant vapor of the amine-absorbing liquid being fed to the reclamation column. In the CO2 chemical absorption system, part of the condensed water obtained by cooling the exhaust gas made free from CO2 in the reclamation column is used as a solvent for an inorganic alkaline solution to be added into the absorption liquid reclaimer so as to remove through distillation the heat-stable salt accumulated in the amine-based absorbing liquid.